1. Field of the Invention
This invention relates to a method of forming a nanolaminate-structure dielectric film and, in particular, to a method of forming a nanolaminate-structure dielectric film in the form of a molecular-layer laminate of strontium oxide (SrO) films and titanium oxide (TiO) films.
2. Related Background Art
Recent semiconductor devices have significantly increased in capacity and, in the DRAM (Dynamic Random Access Memory) field, large-capacity memories of 1 Gbit have been put to practical use. DRAM cells each normally comprise one transistor and one capacitor. Charges are stored in the capacitors as memory information and exchanged by the transistors. Each capacitor comprises two electrodes, i.e. a lower electrode connected to a diffusion layer electrode of the transistor and an upper electrode commonly connected to a reference potential, and further comprises a capacitor dielectric film disposed between those electrodes.
In the large-capacity memory, following the reduction in size of each memory cell, the occupation area of a capacitor portion is also reduced. However, in the DRAM, since charges in the capacitors are used as memory information, each capacitor requires a capacitance equal to or greater than a certain value in order to ensure the stable memory operation. As a method of achieving the capacitance equal to or greater than the certain value in the reduced memory cell area, an attempt has been made to reduce the thickness of a capacitor dielectric film or to use a dielectric material film having a high permittivity as a capacitor dielectric film. For example, in the current state, a film thickness of 1 nm or less is required in terms of a silicon oxide (SiO2) film. For this purpose, practical use is made of a dielectric film, such as an aluminum oxide (herein after referred to as “AlO”; relative permittivity: about 9) film or a Ta2O5 (tantalum pentoxide; relative permittivity: about 50) film, having a higher permittivity than a SiO2 (silicon oxide) film and a Si3N4 (silicon nitride) film which have hitherto been used. Further, practical use of a SrTiO3 (strontium titanate; hereinafter referred to as “STO”) film is also now under review.
As a patent document for the STO film, there is Japanese Unexamined Patent Application Publication (JP-A) No. 2004-146559 (hereinafter referred to as “Patent Document 1”) filed by the present inventor. In Patent Document 1, the present inventor obtains a dielectric film having a high relative permittivity (about 150) by depositing a STO film by a CVD method at 420° C. and then heat-treating it at 500° C. or more and 650° C. or less.